1. Field of the Invention
The invention concerns an electrostatic spraying installation, in particular one for a new type of paint, referred to herein as "water-based" paint, usable on automobile bodies and having low resistivity.
2. Description of the Prior Art
These new paints have aroused considerable interest among users because their use causes very little pollution of the environment. The invention is specifically directed towards solving the problems arising from the low resistivity of the new paints when used in electrostatic sprayers held at a high voltage and where the paint is supplied through the intermediary of long, closed loop, electrically grounded paint circulation circuits.
A conventional electrostatic spray painting installations as used in a production plant as large as an automobile manufacturing plant, for example, generally comprises a plurality of closed loop paint circulation circuits that are very long (these circuits may cross the whole of a part of the plant) and which establish the connection between large paint storage tanks and the various spray booths. It is therefore necessary to provide at least one such circuit per color and another circuit of the same kind for the solvent or dilutant. For obvious safety reasons these circuits are electrically grounded. For convenience they will be referred to as "primary circuits" in the remainder of the description.
In a spray painting booth the objects to be painted, which are automobile bodies in the example under discussion, are carried by conveyor means which pass through said booth and so past a plurality of electrostatic sprayers positioned at different levels and oriented in different directions, and often articulated to their respective supports, so as to paint respective areas of said objects. The size and shape of the objects (especially when they are automobile bodies) are such that at least nine projectors per painting station are normally required. The sprayers are normally offset longitudinally along the length of the conveyor means. However, those skilled in the art have always sought ways to achieve the least possible spacing between the sprayers as the organic solvent paints used until now dry relatively quickly. For the layers or areas of paint applied by different and spaced sprayers to merge in a satisfactory way it is necessary that the areas or layers applied successively remain sufficiently liquid to merge with each other.
Another problem where the introduction of the new paints is causing changes is that of changing the color. In the automobile industry in particular there is no question of painting long runs of automobile bodies the same color. To the contrary, it is usually the case that the color has to be changed for virtually every body. This means that it must be possible to execute extremely fast electrostatic sprayer rinsing and drying cycles. This is generally achieved without too much difficulty in the case of organic solvent paints. If the installation is designed in such a way that changing the color entails an operation to drain-rinse-dry a storage tank which receives paints of different colors, the color change sequence is necessarily slow and constitutes a serious obstable to the functioning of the system.
However, the most serious problem to be solved is that resulting from the low resistivity of the water-based paint itself. If the high-tension voltage is applied directly to the paint spraying member (a spray bowl rotating at very high speed, for example) the high-tension generator is then grounded through the paint itself since this has such a low resistivity that it establishes an electrical connection of negligible resistance as far as the primary circuit.
Various solutions have been proposed for solving or circumventing the problems stated hereinabove.
Thus it has been proposed to ground the sprayer and to apply a high-tension voltage to the objects to be coated. This is described, for example, in the document DEOS 3344703. The invention is not concerned with this type of solution which has major disadvantages and in particular that of making it necessary to maintain the object (an automobile body) at a high potential throughout the spraying time. Also, dust tends to be deposited on the object when it is held at the high-tension voltage.
Other solutions involve not applying the high-tension voltage to the spraying member in contact with the paint. Ionizing electrodes are instead provided near said spraying member. This is the case, for example, with the system described in French patent 1 574 988. The disadvantage of a system of this kind is that the particles of paint leave the spraying member (the rotating bowl) with an electrical charge which attracts them towards the ionizing electrodes. The ions emitted by these electrodes then have to attach themselves to the particles and change the sign of their electrical charge so that they can then move towards the object. It has been observed that certain particles do not have sufficient time for their electrical sign to change during their movement towards the electrodes so that they reach and pollute the latter, the effect of which is to render the insulative support of the electrodes electrically conductive and thus eventually to establish a conductive path between the high-tension supply and ground. Also, these problems sometimes result in the spraying onto the bodies of lumps of paint formed by an agglomeration of paint particles that have reached the electrodes.
The invention is not concerned with this category of solutions either, but attempts to retain the advantages of electrostatic charging of the paint by the spraying member raised directly to the high-tension voltage, that is to say usually by a spray bowl rotating at high speed.
In the same line of thinking it has previously been proposed to insert between the primary circuit and the sprayer storage tanks for "isolating" the electrical connection, that is to say to provide intermediate storage for the paint that can be raised to the high-tension voltage and insulated from ground. For example, U.S. Pat. No. 4,544,570 provides such isolation by atomizing the paint from the primary circuit in an intermediate receptacle. This solution introduces the risk of the paint being soiled or oxidized by the ambient air or thickened by partial evaporation of the water and co-solvents.
French patent 2 572 662 proposes to fill the intermediate storage tank with just the quantity of paint necessary for each application. This operation is conducted in such a way that the paint is protected from exposure to air. However, and as previously mentioned, each color change makes it necessary to clean out the intermediate storage tank and all the pipes connected to it. Also, isolation is re-established after filling by draining and drying a sufficient portion of the pipework on the upstream side of the intermediate storage tank. The time this takes on each color change is prohibitive. French patent 2 454 846 (and the corresponding U.S. patent application Ser. No. 032 790) proposes an arrangement comparable to that just described but requires, for each sprayer, as many intermediate storage tanks as there are colors. In an installation comprising a plurality of sprayers this teaching therefore results in a considerable increase in the number of intermediate storage tanks and thus in the cost of the installation.
The invention is aimed at eliminating all of the problems stated hereinabove and proposes a new arrangement requiring only one intermediate storage tank per color, irrespective of the number of sprayers, and also making it possible to minimize the time to change the color by virtue of a new arrangement of the sprayers relative to each other.